During the last year we have characterized a number of surface antigenic determinants on Chlamydia trachomatis using recombinant DNA and chemical techniques. The gene encoding the major outer membrane protein (MOMP) of C. trachomatis was subcloned into lambda gtll in order to isolate small DNA fragments that expressed epitopes of the MOMP. The subclones were then subjected to DNA sequencing, and the sequences were aligned with the MOMP gene sequence. In this manner we have been able to identify the regions of MOMP that contain type-, subspecies-, and species-specific epitopes. The genus-specific epitope of Chlamydia--which is located on the lipopolysaccharides--was characterized using a combination of recombinant techniques and chemical analysis. Recombinants that expressed the genus-specific epitope were analyzed to determine the structure of their LPS, and this knowledge aided inthe determination of the structure of the chlamydial LPS structure which contains the genus-specific epitope. The nature of the biosynthetic enzyme involved in the expression of the chlamydial genus-specific epitope is currently being analyzed.